Acoustically treated motor



May 12, 1959 v. J. PICQZZI ETAL 2,886,721

ACOUSTICALLY TREATED MOTOR Filed Dec. 3, 1956 2 Sheets-Sheet 1 .gq L

Inventors:

/hcerl' J P700224 6776:16 D spawns); Robert L. Ma,

7776/? Attoryay May 12, 1959 'v. J. PlCOZZl l-n-AL 2,886,721

ACOUSTICALLY TREATED MOTOR I 2 Sheets-Sheet 2 Filed Dec. :5, 1956 s 22.2 v Q.

J I)! o I,

[HI/anions. Vincent JP/cazz/ i/bert 12 5,0002%); Robert L. Wa/A 7776/)- Attorney ACOUSTICALLY TREATED MOTOR Vincent J. Picozzi, Gilbert D. Spooner, and Robert L.

Wall, Schenectady, N.Y., assignors to General Electric Company, a corporation of New York Application December 3, 1956, Serial No. 625,896

6 Claims. (Cl. 310-51) The invention described herein relates to dynamoelec- 'tric machines and more particularly to an acoustically treated motor designed for reducing the overall noise level and quality of noise emanating from the machine during operation.

A persistent problem encountered in design of dynamoelectric machines is one that has nothing to do with the efficiency or life of the machine itself, but has much to do with the efficiency and comfort of the worker. That problem is noise with its accompanying undesirable efiects of fatigue, irritation and exhaustion. It is well known that electrical machines produce unwanted noises at many different frequencies and manufacturers generally have not directed their efforts toward treatment of the noise With a view toward reducing or eliminating it since the machines are usually located in areas where the noises produced by adjacent machines are considerably higher so that it is substantially useless to attempt reduction in the noise level in a particular machine in the group. Likewise, when a single machine is located in an area not requiring the continuous presence of an operator, the expense involved in providing the machine with acoustic insulation is not justified by the limited amount of contact by an operator with the machine. However, the upward trend in noise hygiene in industry dictates the need for design and development of sound absorbing structures, which when incorporated in a machine, are capable of eliminating the adverse efiects caused by noise.

Sound insulation of mechanically operating equipment is not new as indicated by the reduction obtained in noise levels of internal combustion engines, for example, by attaching glass wool mats to the inner surfaces of a cover which serves to absorb the varying sound frequencies generated by the moving engine parts. Reduction of noise levels in apparatus utilized in other fields has been accomplished, but to our knowledge, no electrical machine has been designed incorporating sound damping features prior to development of this invention.

Accordingly, an object of our invention is to provide a sound insulated motor capable of damping the noises produced by the rotating, magnetic and vibratory parts prior to the issuance of such noises from the machine.

Another object of the invention is to provide an improved air system for preventing the entry of oil inwardly along the rotor shaft during operation of the machine.

In carrying out the objects of our invention, we surmount a dissipative type mufiier on a motor consisting of a plurality of sound insulated plenum chambers having access to the inner confines of the motor and being located in a path followed by circulating air used in ventilating the machine. A rotor mounted fan draws air inwardly through louvered openings in a side of the mufiler unit and into sound insulated chambers prior to being circulated States Patent through internal passages in the motor and finally into the mufiler discharge chambers where it is caused to follow a tortuous path before ejection through openings on the ends of the machine. The machine is further designed with an air deflector located in the air circulating path and arranged to encompass a portion of the rotor fan so as to provide an air pressure to the machine bearings for preventing movement of oil from the bearings along the rotor shaft and into the machine.

While the specification concludes wtih claims particularly pointing out and distinctly claiming the subject matter which we regard as our invention, it is believed the invention will be better understood from the following description taken in connection with the accompanying drawings in which:

Figure 1 is a view in elevation, partly in section, showing the operating parts of a motor and details of a mufiier utilized in absorbing sound noises generated by the machine; and

figure 2 is a perspective view, partly in section, illusrating the tortuous paths the air must follow in ventilating the machine.

The sound levels or noise produced by a given motor in a certain installation is determined by the latent ability of the motor to produce noise and also by its acoustic environment. The latent ability to produce noise is generally defined as the sound power in watts. Sound power is essentially independent of environment and the distance factor usually associated with noise measurements is not considered. To determine sound power, it is necessary to determine the average sound intensity acting on an imaginary area that completely encloses the machine. In practice, this imaginary area usually takes the form of hemisphere, the center of which is coincident with the geometrical center of the motor so that all of the noise measurement points lie on this hemisphere.

As to environment, a sound level produced by a motor in free space or in an acoustically black enclosure will decrease with increasing distance from the motor at a rate determined by the inverse square law. The noise will be halved each time the distance is doubled from the source. Conversely, if the motor is installed in a reverberant or acoustically white enclosure, the inverse square law may not apply. Under the latter conditions, there is so much reflected sound energy that a diflfuse field is formed when the sound level is independent of distance. The net result is that at any distance from the motor the sound level may be much higher than it would be at the same point if the reflections were not there. One method of reducing noise in an area adjacent a dynamoelectric machine is to locate the source of noise at a sufiicient distance so that the sound levels are reduced by the effect of distance. However, where the room or enclosure is reverberant, this concept no longer applies because the sound level is essentially independent of distance. Correction of this condition can be made by employing a still quieter operating machine or by adding sound absorbing material to the enclosure for reducing these reverberation effects.

The noise produced by dynamoelectric machines, such as an induction motor, which hereafter will be utilized in describing the invention, is generally quite complex and is composed of many frequencies. Overall sound pressure level measurements usually do not take into account the frequency content of the noise. To adequately describe the noise, it is necessary to determine this frequency content and an octave band frequency analyzer used in conjunction with a sound level meter makes it possible to determine a sound pressure level in each of the eight standard frequency bands extending from 37 /2 to 9600 cycles per second.

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown a motor symmetrical about both ends and enclosed within a divided casing consisting of a combined frame and enclosing portion '12 and an upper portion or muffler unit .14. The lower portion 12 is generally of box-like configuration having split end shields on opposite ends thereof and being made of a material of sufficient strength to support a rotor shaft centrally positioned therein. Mounted within the casing portion 12 and supported in a cast iron frame .is a stator 18 made of a plurality of silicone sheet steel punchings 20, each being enamel insulated and the entire "stack of punchings tightly pressed together to prevent loosening during operation of the machine. The stator is provided with form wound coils.22, including projecting end turns 24, covered with a coating of insulation compatible with the type of service furnished by the motor. The stator is further designed with a plurality of outwardly directed radial openings 26 utilized in ventilating and carrying away the heat generated by the machine during operation.

The rotor 28 is likewise constructed from a plurality of l'siliconesteel punchings and the conductors are positioned in slots provided therein and terminate in end rings as is the usual custom in squirrel cage rotor construction. Nonmagnetic .steel'retaining rings 30 are shrunk onto the symmetrical ends of the shaft and over the entire end ring assem'blies to prevent buildup of stresses in the end ring and ing losses and increases the life of lubricating oil utilized 1 :in'lubricating the bearing and carrying away generated .heat. The bearing 38, in which the journal is adapted for :rotation, is electrically isolated from the frame of the machine and is of a split sleeve type lined with hard tin ibabbittcast into the bearing sleeves and large dovetail an chor grooves assuring a good bonding action thererb'etween. Asmore clearly shown in Figure 1,.ribs 39 providedinzeach'ofthe end shields 16 areequipped with cored passageways'40-utilized.in carrying airfromthe discharge :side of the fanto the bearingsand for purposes as hereinand the other, the discrete frequency noise, that is, indivi'dual'noises of a single frequency, caused by the fundamental blade frequency and its harmonics. The first type tof'noise, being miscellaneous and random, and usually in the low frequency range, can beaccepted when acoustic consideration is given to a motor. :is more troublesomeand is overcome in the present in- The discrete noise ventionby increasing the number .of rotor fan blades 32 from. four toamanyas twenty-one or twenty-three. This change results in noises ofhigher frequency, the am- 1..plitude\ of which is-easierto control by. muffiing.

.Thelsiren effect: in theradial, rotor, and stator. ductsof the type-disclosed herein and used for ventilation purposes :mamfestsitself inaipure tone that occurs at a frequency :equal to the number ofrotor bars times .the running speed l quency, but it has been found that it also can be considerably reduced by mufliing.

Magnetic noise in electric motors is associated with the vibration of the motor frame, or its components, where the vihramotive force is supplied by the magnetic forces within the machine. In large machines, force waves that are apt to cause the most objectionable noise are associated with the number of rotor bars times the running speed. Knowing the number of poles and the magnitude of the force waves, they can all be considered in the initial design of the machine, and it is possible to choose a combination of rotor and stator slots so that the force waves produced will have a'large number of poles and thereby be ineffective in causing the stator and the frame to produce noise.

Mechanical noise is that associated with 'runningspeed noise and its'harmonics. Generally, it occurs at fairly low frequencies and can be attenuated by the use of muffiing elements located between the source and exterior portions of the machine.

In order to substantially reduce or eliminate the noises generated by the machine and falling within the three classes generally outline-above, the number of fan blades 52 were increased from four to approximately twentythree, as mentioned above, so as to increase the basic :sound .frequencies emitted by the blades. Such an increase in blade number provides a high frequency sound that can be absorbed more readily by sound absorbent material. The-other aerodynamic noisesand magnetic noises are:susceptible of being reduced especially since the siren :noises caused by interaction between the rotating rotor andzstator slots is already at a high frequency.

The new structure employed for obtaining such a reduction-in ibothzthe sound and particular frequency levels .re- :sides in the upper casing 14 mounted above the motor but in actual contact with casing .12.

The cas'ing.14 comprises .a mulfier unit and consistsof :aplurality .of identically formed compartments or plenum chambers 42 positioned on opposite sides of the machine 'for carrying the ventilating air and absorbing noises generated .during operation. As shown in Figure 2, each plenum chamber is formed by ribs 44. These ribs are arranged for contact with structural beams 46 integrally formed 1111 the lower casing so as to provide a baseton :which the ,uppercasing14 rests. The sides are equipped with .louvered openings 48 for entry of ventilating air whilethe ends are furnished with alike set of openings .50 for discharging the. air endwise of the machine.

Each plenumchamber 42.is thoroughly insulated with .abasecoating ofa mastic damping compound 52-or similar substance to reduce the amplitude of the vibration if za mechanical resonant condition should exist. An intermediate insulation 54 of fiberglass blanket or sound absorbing material-is placed over the mastic compound :covered surfaces and anchored to the casing walls by bolts :55 or similar securing means. Wire mesh 58 is superposed on the intennediate insulator 54 for the pur- :pose ofincreasing the'mechanical strength of the system .and :reducing its susceptibility to air erosion. The arrows applied to each of these figures illustrate the path of cooling airthrough the motor and it is to be noted "that by 'virtue of the arrangement of openings 48 and :50 in rthemufller unit ,14, there ,is no direct path-for noises generatedin the machine to escape to the atmosphere. .Inother words, the various sound'noises'must .bounce .or reflect ,from the insulated .walls of the plenum chambers before'being emitted to theatmosphere.

.Asplit air deflectori56is provided on each end of the :machine 'lforpseparating. the inlet vair from the: discharge :side ;of:.the fans. Since the; pressure at the inletis considerably lessthanthat at the outlet,'the,deflector makes possible the conveying of high pressure air from chamber "70rthrough rib-41 and passagewayn40 to opposite sides of .bearingr38. The; principal function employed by this'air ..1s to rprevent -.the seepage .of ;oil.; from ,thebearing alone the rotor shaft 34 and into the machine. It will be noted that in each of the figures shown, the high pressure air is directed against the shaft through orifices 72 on opposite ends of the bearing. The seal thus provided consists of an oil seal, as is usual in this type of bearing, supplemented by an air seal which actively serves to prevent transfer of oil from the bearing into the working parts of the machine. Further, since the passageways 40 are formed in the structural portions, alignment of the openings is automatically achieved when the parts are installed in position. This eliminates the need for providing additional air carrying pipes and simplifies the removal of end shields 16 when maintenance on the machine is necessary.

The mufiler unit 14 is open at its bottom end, with exception of ribs 44, for receiving the air circulated to and through the machine. The several advantages offered by the mufiler unit described above is that air is permitted to enter the mufiler unit from an aisle where a machine or plurality of machines may be located and then discharged along the line above the machines where it does not come in contact with an operator stationed adjacent thereto. By virtue of the series of chambers which open into the machine, the mufller unit can be removed in a single operation exposing the various parts of the machine so as to facilitate maintenance and repair and for cleaning the machine, particularly when operated in dust laden atmospheres. Another distinct advantage is that the air passageways 40 are provided in a rib of the split end shields thereby eliminating the necessity of providing pipes necessary for conveying air from the high pressure source to the bearings.

In operation, air is drawn inwardly through louvers 48 provided in the side of the muffier unit 14 and passed inwardly through openings provided in the bottom of the plenum chambers and into the fan 32 as shown by the arrows in each of these two figures. As the rotor revolves, fan 32 generates an air pressure which directs the incoming air through the stator and into the muffier unit 14 where the noises gathered during its path of travel through the motor are absorbed by the insulation in the mulfier unit. As the air enters the mufiler unit, it is caused to travel through a tortuous passage prior to ejection from the casing through openings50 provided on the ends thereof.

In view of the above, it will be evident that many modifications and variations are possible in light of the above teachings. For example, any type of insulating material can be used in the plenum chambers provided in the mufiler unit 14 and the chambers themselves can assume a size, shape or configuration compatible when providing sound absorbing characteristics.

It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An acoustically treated motor for reducing the sound level of noises generated therein comprising a casing enclosing a stator and a rotor arranged for electrodynarnic operation therewith, a shaft on said rotor positioned in bearings provided in oppositely disposed removable end shields on said casing, fan means on said rotor imparting a pressure to air utilized in cooling the motor, a muffler unit having ribs supported by frame members in said casing, said ribs defining a plurality of interconnected acoustically lined plenum chambers having direct access to the exposed internal portions of said motor, and an air inlet and outlet positioned in adjacent walls of said unit respectively communicating with the fan means and plenum chambers, whereby aerodynamic, magnetic and mechanical sound noises carried by the cooling air and radiated into the plenum chambers are attenuated by the lining thereon, thus minimizing the sound level of noises emanating from the machine.

2. A motor for reducing the level of self-generated sound noises comprising a casing having a lower portion constructed to contain a stator and a rotor for said motor, a shaft extending outwardly from each end of said rotor and being arranged for positioning in a pair of bearings mounted in removable end shields on said casing, fan means mounted on each end of said rotor capable of providing an air pressure for ventilating said motor and for providing a pressure to opposite ends of said bearings for preventing the seepage of oil into the working parts of said motor, and an upper casing detachably mounted on said lower casing and being provided with a plurality of insulated plenum chambers for absorbing sound noises generated by said motor during operation, said casing comprising an air inlet located on a side of said upper casing and an outlet located on the ends thereof, said upper casing having access to the inner confines of said motor so that upon removal of said upper casing as a unit, the operating parts of said motor are exposed for maintenance and repair, and said plenum chambers in said upper casing being lined with an insulating material of a type capable of absorbing the aerodynamic, magnetic and mechanical sound noises generated by said motor during operation.

3. An acoustically treated motor comprising a casing enclosing a stator and a rotor having fans on opposite ends thereof for circulating ventilating air through the motor, a muffler unit detachably mounted on said casing, said unit comprising side, end and top walls forming an enclosure open at the bottom so that upon removal of said unit, the operating parts of the motor become fully exposed for maintenance and repair, a plurality of ribs in said unit defining interconnected plenum chambers, said ribs being positioned for support by frame members in said casing and simultaneously serving as air directors for channeling ventilating air to the plenum chambers, an air inlet and outlet respectively located in the side and end walls of said unit for permitting ventilating air to be drawn inwardly through the inlets by the fans and circulated through the motor for cooling heat producing parts and for picking up aerodynamic, mechanical and magnetic noises generated by the motor during operation, and sound absorbing means lining the walls of said plenum chambers effective in attenuating said noisw radiated by the motor and carried thereinto by the ventilating air as the latter traverses the plenum chambers prior to discharge from the mufiler unit.

4. The combination according to claim 3 wherein said mufller unit is symmetrical on both sides of a vertical plane passing axially through said motor.

5. The combination according to claim 3 wherein said ribs are positioned with respect to each other in said muffier unit so that noises carried by said air or radiated into the mufiler unit are required to contact the insulated Walls of the plenum chambers prior to being discharged from the motor.

6. An acoustically treated motor comprising a stator and a rotor having a shaft supported in removable end shields, fans on said shaft for circulating ventilating air through the motor, an open topped casing enclosing the stator and rotor and having vertical frames providing a plurality of openings in direct communication with the operating parts of the motor, a detachable mufiler unit mounted on said casing and being closed on all sides except for air inlets and outlets respectively positioned in the side and end Walls and being open at the bottom so that upon removal of the unit, the operating parts of the motor are exposed for maintenance and repair, said unit comprising internally disposed ribs defining plenum chambers coextensive with said openings, sound absorbing means lining said plenum chambers for attenuating aerodynamic, magnetic and mechanical noises radiated or carried thereinto by the ventilating air, and a parallel air flow path for air discharged by said fans, one of said paths being formed by deflector shields encompassing.

7 said vfans, bored openings in said end shields communieating at one end with the pressure side of the fans'and terminating at the other end in seal means surrounding bearings sugporting said shaft, so that pressurized air from the fans acting on the seal means confines oil in said bearings, the other of said air flow paths extending upwardly to the plenum chambers wherein the air is required to follow a tortuous path prior to being discharged through said outlets.

UNITED STATES PATENTS Smith Feb, 10, 1942 Bentley Sept. 17, 1946 Kent Jan. 17, I956 FOREIGN PATENTS I Great Britain of 1912 

